1. Field of the Invention
This invention relates to a process for producing a phosphor. This invention particularly relates to a process for producing various kinds of phosphors with a urea melting technique.
2. Description of the Related Art
Phosphors to be utilized for cathode ray tubes, X-ray intensifying screens, and the like, have heretofore been obtained with techniques, in which powdered raw materials are mixed together, the mixture of the powdered raw materials is introduced into a firing vessel, such as a crucible, and heated at a high temperature for a long period of time, a phosphor is thus formed with a solid phase reaction, the thus formed phosphor is pulverized with a ball mill, or the like, and the thus obtained phosphor particles are subjected to classification.
However, with the techniques utilizing the solid phase reaction, it is necessary for the mixing to be performed for a long period of time, and it is necessary for the firing to be performed for a comparatively long period of time even at a high temperature. Also, the techniques utilizing the solid phase reaction have the problems in that the powdered raw materials are not always capable of being mixed uniformly, and in that a phosphor having reliable quality is not always capable of being obtained. Further, during the pulverization processing performed after the solid phase reaction, the phosphor is subjected to physical and chemical impacts. Therefore, the problems occur in that defects occur within the phosphor particles and on surfaces of the phosphor particles and cause light emission intensity of the phosphor to become low. Furthermore, with the techniques utilizing the solid phase reaction, wherein the powdered raw materials are heated at a high temperature for a long period of time within the firing vessel, such as the crucible, the problems occur in that impurities enter from the firing vessel into the phosphor and cause the light emission intensity of the phosphor to become low. The problems also occur in that the solid phase reaction does not progress sufficiently, depending upon the particle size of the powder raw materials, and impurity phases are mixed in the obtained phosphor and cause the light emission intensity of the phosphor to become low. Further, with the techniques utilizing the solid phase reaction, wherein the powdered raw materials are heated at a high temperature for a long period of time, the amount of energy required is not capable of being kept small, and the cost of the phosphor is not capable of being kept low.
Besides the aforesaid techniques utilizing the solid phase reaction, techniques utilizing liquid phase reaction, such as a pyrolysis technique, a co-precipitation technique, and a metal alkoxide technique, have also been known. With the techniques utilizing the liquid phase reaction, a solid-state intermediate product is obtained from a uniform mixed solution of soluble constituents and is then subjected to heat processing. Therefore, the techniques utilizing the liquid phase reaction have the advantages over the techniques utilizing the solid phase reaction in that synthesis is capable of being performed at a comparatively low temperature, and in that the quality of the obtained phosphor is uniform. However, the techniques utilizing the liquid phase reaction have the problems in that complicated production processes are required, and therefore there is a high possibility of impurities being mixed into the obtained phosphor. The techniques utilizing the liquid phase reaction also have the problems in that limitation is imposed upon the kinds of the raw materials.
A sol-gel technique has also been known. With the sol-gel technique, a metal alkoxide is used as a raw material, and a sol, in which fine particles of a metal oxide or a metal hydroxide have been dispersed, is obtained with hydrolysis and polymerization reaction. The reaction is then allowed to progress even further in order to obtain a gel, the thus obtained gel is fired, and fine particles of a metal oxide are thereby obtained. However, the sol-gel technique has the problems in that complicated production processes are required, and therefore the technique is not appropriate for mass production. Also, the sol-gel technique has the problems in that a precursor is unstable and is difficult to processing.
Further, a spray drying technique has been known. With the spray drying technique, a solution containing metal elements acting as constituents of a phosphor is sprayed into a carrier gas by use of an ultrasonic nebulizer, or the like, and fine droplets of the solution are thus formed and dried into metal salt particles or metal complex particles. The metal salt particles or the metal complex particles having thus been obtained are then subjected to pyrolytic synthesis, and the phosphor is thereby obtained. However, the spray drying technique has the problems in that the quality of the obtained phosphor is not capable of being kept high due to the presence of pores within the powder.
As one of techniques, which have the advantages of both the solid phase technique and the liquid phase technique, a burning technique has heretofore been known. With the burning technique, after a metal nitrate and urea have been mixed together, the resulting mixture is heated to a comparatively low temperature (approximately 500° C.) in a reaction furnace, and a decomposition gas is burned. Powder of a composite oxide is thus obtained quickly and easily. However, with the burning technique, burning conditions vary in accordance with a rate of heating of the mixture, a mixing ratio between the raw materials, a shape of the reaction furnace, a volume of the reaction furnace, and the like, and therefore quantitative synthesis conditions are not capable of being obtained. Also, the burning technique has the problems in that uniform burning is not always kept, and therefore the quality of the obtained product is not capable of being kept at predetermined quality.
A technique for producing a composite oxide, which has reliable quality, at a low cost and efficiently is described in, for example, U.S. Pat. No. 6,274,110. With the technique described in U.S. Pat. No. 6,274,110, a metal nitrate or a solution containing the metal nitrate is mixed with urea or carbohydrazide, the resulting mixture is heated to a temperature falling within a range such that the mixture does not become ignited, a composite oxide precursor is thus prepared, and the composite oxide precursor is heated.
However, the higher order compound described in U.S. Pat. No. 6,274,110 is the composite oxide alone. Also, the composite oxide, which is obtained with the technique described in U.S. Pat. No. 6,274,110, is constituted of the powder composed of aggregates of particles having irregular shapes. Therefore, in cases where a phosphor slurry containing the composite oxide is coated in order to form a phosphor film, a heterogeneous phosphor film having a low packing density is obtained. The thus obtained phosphor film exhibits a low light emission intensity. In cases where coarse particles of the composite oxide are removed with a classifying operation, the sizes of the particles of the composite oxide may be capable of being trued up to the particle sizes falling within a certain range. However, the classifying operation is of bad workability and causes the yield to become low.
Also, as for phosphors containing a silicate as a principal phase, the phosphors have heretofore been produced with the techniques utilizing the solid phase reaction. With the techniques utilizing the solid phase reaction, it is necessary for the raw materials for the phosphors to be fired at a temperature as high as at least 1,500° C. Also, the techniques utilizing the solid phase reaction have the problems in that the raw materials are not always capable of being mixed uniformly. Further, during the pulverization processing performed after the solid phase reaction, the phosphor is subjected to physical and chemical impacts. Therefore, the problems occur in that defects occur within the phosphor particles and on surface of the phosphor particles and cause light emission intensity of the phosphor to become low. In U.S. Pat. No. 6,274,110, Si is described as a metal, but nothing is suggested as for a production example. This is presumably because an Si source dissolved in urea melt had not been found out.